It is known from many studies that space heating inside buildings using infrared rays, or otherwise known as radiant heat, provides energy savings over conventional natural or forced convection heating methods. The more a radiant heater can provide radiant heat versus convective heat, the more the savings are. In this day and age, where there are great requirements for reducing energy consumption, radiant heating is of the essence as space heating is one of the major contributors of energy use. There are basically three reasons for the energy savings with radiant space heating: One of them being that since infrared rays do not heat the air, but rather objects, the vertical air temperature profile in the room is much more uniform, thus causing less heat loss to the outdoor near ceiling level. Secondly, having a more uniform vertical temperature profile in the room causes the attendees to feel more comfortable, a lesser feeling of cold feet, hence resulting in thermostat set point to be set lower than would otherwise be the case. Thirdly, being heated with infrared rays similar to the sun to a certain extent, people feel warmer than they normally would at a specific air temperature, which again results in thermostat set point to be set lower than would otherwise be the case.
There are several methods being used today to provide infrared heating from an apparatus, but few of them make their way into the home, office, or commercial spaces; and those that currently do, have low radiant heat efficiencies. Few of them, also, make their way into warehouses, industrial and commercial buildings in high ceiling (above approx. 3.7 meters (12 feet)) areas; and for outdoor spot heating. For the latter applications, high intensity radiant heaters are required and they must have high radiant heat efficiencies for the user to benefit from all of the input power. Radiant heat efficiency used here is defined as:Radiant heat efficiency (%)=(Radiant heat output)/(Power input)×100
The total radiating power emitted from a hot surface is defined by the Stefan-Boltzmann law:P=σεAT4, where:P is the radiant power in watts,σ is the Stefan-Boltzmann constant=5.6704×10−8 W/m2/° K4,ε is the emissivity of the radiating surface,A is the area of the radiating surface in square meters, andT is the temperature in degrees Kelvin of the radiating surface.This equation shows that the radiating power from a hot surface is proportional to the fourth power of its temperature in degrees Kelvin, and to the single power of its emissivity and of its area. Thus, it is greatly advantageous to have as high a temperature as possible on the radiating surface of the heating element, in order to have a smaller heating device, which will have less area for generating convective heat. Any inefficiencies in radiant heat from an apparatus, converts mostly into convective heat. Hence as explained previously, the higher the radiant heat efficiency a heating apparatus can provide, the higher the energy savings shall be for the user in heating his home, office, commercial space, industrial space, or outdoor patio.
The most efficient methods of providing radiant heat today, are those apparatuses that make use of a cylindrical or tubular heating element, along with a reflecting surface at the back and around the main axis of the element, which reflects infrared rays in the desired direction. These include heating elements with quartz glass tubes. This type of design permits the use of higher temperatures of the heating elements. In order to have a high radiant efficiency, the heating element needs to either: have a high coefficient of emissivity like a ceramic heated tube, or be an infrared emitting quartz lamp. Unfortunately, infrared emitting quartz lamps and ceramic heated tubes are fragile and their life expectancies are approximately 5 000 hours and 10 000 hours, respectively. These kinds of heating devices generate light or glow red when in operation, and the heat provided is much too concentrated and is annoying for the user standing a few feet away. Gas-fired heated tube radiant heaters are similar in basic design, except they are much larger, they do not glow red, operate at lower temperatures, and yield much less radiant heat efficiency. Gas-fired heated tube radiant heaters have low radiant heat efficiencies, because of the larger surface area of the heated tube, giving off excess convective heat, and mostly because a high percentage of heat is lost with the exhaust flue gases. In addition, all of these kinds of devices are generally difficult to adapt to meet the safety standards of certifying agencies, for use indoors in low ceiling (below approx. 3.7 meters (12 feet)) spaces such as in homes, office and commercial spaces. This is because of the intense radiant heat and convective heat generated for installations close to a wall and/or a ceiling. They are mostly used in high ceiling spaces such as warehouses, industrial or commercial buildings, and outside patios for spot heating. All of these types of heaters require a reflecting surface with a high total reflectance, or low coefficient of emissivity, to reflect a high percentage of the incoming rays in the direction the heat is required. Depending on their specific design, the radiant heat efficiencies of these types of heaters today are known to be the following in Table 1 below:
TABLE 1Tubular Heating Elements Used in Radiant Heaters with Reflectorsand their Radiant Heat EfficienciesHeating Apparatus TypeRadiant Heat Efficiency Range (%)Quartz lamp80%-90%Quartz tube55%-65%Ceramic tube83%-93%Metallic tube50%-75%Gas-fired heated tube40%-55%
Another group of radiant heating apparatus being used today, makes use of a heating element of some kind to heat a flat or curved panel, which in turn provides the radiant heat to the user and space. These radiant panels are typically located on the ceiling, flush within a T-bar suspended ceiling, suspended from the ceiling, on a wall, attached to a wall, suspended from a wall, flush with the floor, inside the floor, or below the subfloor. The front faces of the panels are made of metal, or glass, or stone, or gypsum board, or mortar, or concrete, or plastic, or solid ceramic material, or ceramic fiber material, or mesh metal in the case of gas-fired heaters. The panels are heated with heating elements or gas burner nozzles that are inserted inside or at the back of the panel. Some of these panels have heat insulation material at the back to prevent a large amount of heat to go in the wrong direction. Some other panels do not have heat insulation at the back, in order to use this back heat as convective heat which in turn rises to the ceiling, as do the convective types of heating apparatus. Unfortunately, radiant panels, because of their basic flat design, cannot exceed much more above 50% in radiant heat efficiency. The infrared rays radiating at the back of the hot panel cannot be redirected to the front, and are thus converted into convective heat. Adding thermal insulation at the back of the panels increases the temperature at the front, whereby adding to the radiant heat generated to the occupants. Some of these types of radiant heating devices are being used for heating home, office and commercial spaces having low ceilings (below approx. 3.7 meters (12 feet)); and other types are being used for heating warehouses, industrial buildings, commercials buildings having high ceilings (above approx. 3.7 meters (12 feet)), and a few others used for patio spot heating. Because of their location in a building, heating panels used for space heating have limits on the temperatures they can attain to meet the safety standards and prevent fire hazards.
One can easily calculate and estimate the radiant heat efficiency of heating elements using the Stefan-Boltzmann equation, rearranged for the transfer of energy between the heating apparatus and the surroundings; and also estimate the radiant heat efficiency of the radiant heater by using the percentage total reflectance of the reflector, and the angle of direct emission of the heating element towards the surroundings. These can be calculated for panel type radiant heaters and for heaters using opaque tubular heating elements. Here are the equations: